The present invention relates generally to the field of ink-jet printing and, more particularly, to the delivery of ink and the control of ink pressures in ink-jet printheads.
Ink-jet printers have gained wide acceptance. These printers are described by W. J. Lloyd and H. T. Taub in "Ink-Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R.C. Durbeck and S. Sherr, Academic Press, San Diego, 1988) and by U.S. Pat. No. 4,490,728. Ink-jet printers produce high quality print, are compact and portable, and print quickly but quietly because only ink strikes the paper. The major categories of in- jet printer technology include continuous ink-jet, intermittent ink-jet, and drop on demand ink-jet. The drop on demand category can be further broken down into piezoelectric ink-jet printers and thermal ink-jet printers. The typical thermal ink-jet printhead has an array of precisely formed nozzles attached to a thermal ink-jet printhead substrate that incorporates an array of firing chambers that receive liquid ink (i.e., colorants dissolved or dispersed in a solvent) from an ink reservoir. Each chamber has a thin-film resistor, known as a "firing resistor", located opposite the nozzle so ink can collect between it and the nozzle. When electric printing pulses heat the thermal ink-jet firing resistor, a small portion of the ink near it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to form on the paper as the printhead moves past the paper.
Ink delivering systems for conventional ink-jet printheads deliver ink at a slight vacuum, known as a "back pressure", so that the ink does not leak out of the nozzles. Typically, this slight vacuum is approximately two to three inches of water below atmospheric pressure. The back pressure can be created by positioning the ink reservoir below the printhead so that the system equilibrates with a slight vacuum inside the printhead. Alternatively, a slightly negative back pressure can be created using a spring to pull a bladder membrane outward to create a slight negative pressure inside the ink reservoir. This approach is described in U.S. Pat. No. 4,509,602 entitled "Ink Reservoir With Essentially Constant Negative Back Pressure", issued Apr. 2, 1985 and assigned to the assignee of this invention.
Today most conventional ink-jet printheads have an "onboard ink reservoir". In other words, the ink reservoir is physically attached to the printhead and moves with it during printing operations. As the printhead and the ink reservoir move back and forth across the page, the ink accelerates and decelerates and consequently develops pressure surges that can deprime or discharge ink from the printhead. Some previously known onboard ink supplies have a block of foam in the ink reservoir to create the back pressure through capillary action and to prevent the ink from sloshing and developing pressure surges. The foam occupies a large fraction of the ink reservoir volume and thus reduces the capacity of the ink reservoir.
Some ink-jet printheads have "off-axis ink reservoir systems". These systems use a small flexible tube to transport ink from a stationary ink reservoir to a moving printhead. When the supply of ink is low, the user replaces only the ink reservoir. Like onboard systems, acceleration and deceleration of the printhead and the flexible tube create pressure surges that can either deprime or discharge ink from the printhead.
The relative heights of the printhead and off-axis ink reservoir influence the back pressure of the ink-jet printhead. Many previously known systems set the back pressure by using a wide and shallow reservoir placed at a height to produce a slightly negative pressure in the ink-jet printhead. Since the reservoir is shallow, its level does not change much and the back pressure of the ink-jet printhead does not change much. The problem with this arrangement is that tilting the printer can disrupt the operation of the printhead. Another problem is the low ink capacity of a shallow ink reservoir.
One off-axis ink reservoir system is described in Japanese patent document no. 63-256451 (Japanese Serial No. 62-91304) by Kurashima published Oct. 24, 1988.